Assuming it's not loose, it can be adjusted stronger by bending. But be careful, they break easily. But... Depending on which spring it is (your desciption isn't too clear), the octave mechanism may be involved. If the octave mechanism is involved, go to a technician.
Assuming the octave mechanism isn't involved, and it's not loose in the post, unhook it and increase the bend away from the post, but make sure you support it so there's no bending where it goes into the post. In Stephen Howard's sax manual he suggests using round nosed pliers and twisting the spring at many points along the length to get a smooth curve. The twist should be done so that it doesn't bend the spring at the post. You may need to do some dismantling to do this...
But... Why did the spring become weak? Assuming it was OK, something changed and you need to find out what it was. Anything bent/sticking? Missing cork(s)? Springs don't usually loose their tension. Could still be that you caught it cleaning, or on something else.
If you're unsure, take it to a technician! It'll probably be cheaper and a lot less hassle than breaking something.